The present invention relates to a method for detecting the thickness of the walls of hollow manufactured bodies, including but not limited to elongate hollow manufactured bodies, having cross sections of any shape, such as tubes, air intakes for engines, hollow shafts for boats, and the like. The method can be used on manufactured bodies made from non-ferromagnetic materials such as carbon fiber composites, aluminium, wood, and others.
The present invention was developed in the field of aircraft manufacture, for the purpose of measuring the thickness of the walls of elongate hollow bodies used in the manufacture of beams made of fiber-reinforced curable thermosetting material (known as “composite material”), such as wing spars and fuselage longerons. The use of elongate hollow bodies of the aforesaid type is described in U.S. Pat. No. 5,454,895 which discloses a method for manufacturing a composite box structure. The method requires the provision of hollow bodies of elongate shape (called “plugs”), made from composite material, which are enclosed in an impermeable tubular bag. In order to monitor the temperature variations in the composite material while it is polymerized in an autoclave, it is necessary to know the thermal inertia, and consequently the thickness, of the walls of the hollow bodies at various points. The hollow bodies generally have a rectangular or trapezoidal cross section and a considerable length (typically 10-12 m). Their outer surfaces are milled to make them as smooth and accurate as possible, but their inner surfaces, of irregular shape, are not known, and therefore the thickness of the wall of the plug is not known.
The thickness of the wall of a hollow manufactured body of the aforesaid type cannot be measured precisely by using ultrasonic instruments; the raw composite material from which the plug is made has a degree of porosity such that the ultrasonic signal is dispersed and measurement becomes unreliable. There has also been a proposal to detect the thickness of the walls by measuring the Foucault currents induced in a conductive plate which is kept in contact with the inner surface of the wall. This means that the plate must be of considerable size, and therefore tends to bear on the crests of the inner surface, thus falsifying the result.